1. Field of the Invention
The present invention relates to organic electroluminescent display devices generally, and more particularly to an organic electroluminescent display device having an anode covered with a surface reforming layer made of a polymer.
2. Description of the Prior Art
Organic electroluminescent display (OLED) devices have begun replacing cathode ray tube display (CRTD) and liquid crystal display (LCD) devices, because OLEDs typically offer a thinner profile, wider viewing angle, lighter weight, faster response time, and lower power consumption, than CRTDs and LCDs. Another advantage over CRTDs and LCDs is that the relatively simple structure of an OLED, typically an anode, a cathode, and an organic (thin film) emission layer positioned therebetween, permits OLEDs to be easily fabricated using relatively inexpensive manufacturing processes.
The organic emission layer may include several layers depending on its functions. A typical organic emission layer includes a hole injection layer, a hole transporting layer, an emitting layer, an electron transporting layer, and an electron injection layer. The conventional anode electrode is made of a transparent conductive material, such as indium-tin oxide (ITO), having superior transparency and conductivity and a high work function. However, an anode electrode formed of pure ITO generally has an unstable chemical structure. This unstable chemical structure may cause impurities such as oxygen, indium, etc., to be diffused into the organic emission layer, which is typically formed on the anode electrode. This permeation of impurities greatly impairs operability and reduces the lifetime of the OLED.
Another disadvantage of using a conventional, and chemically unstable, anode electrode is that its hydrophilic properties are strong. Consequently, it is difficult to remove accumulated moisture from the interior of the OLED. Additionally, conventional anode electrodes typically have poor interface properties because their surface energy differs from that of the organic emission layer.
One of the methods for improving the properties and lifetime of an organic electroluminescent display device is to reform its anode electrode by covering the anode electrode with what is called a reforming layer, typically a polymer. Exemplary types of conventional polymer layers and conventional methods for forming them are more fully described in U.S. Pat. Nos. 6,127,004, 6,208,075, and 6,208,077. To enhance the hole injection and to improve the operating stability of an organic electroluminescent display device, a conventional reforming layer, e.g. an amorphous CFx polymer layer substantially formed of carbon and fluorine or a Teflon-like polymer, is plasma-polymerized by introducing CFx or CHyFx gas between the hole injection layer and the transparent conductive anode electrode, typically formed of ITO. Additionally, Y. Qiu, Y. Gao, L. Wang, D. Zhang et al. have reported that a fluorine-based polymer formed of CFx or CHyFx can be formed as a reforming layer using a chemical vapor deposition (CVD) method (see Synth. Met. 2002, 130, 235-237). Similarly, S. Chua, L. Ke, R. S. Kumar, K. Zhang et al. have reported that poly(p-xylylene) can be formed as a reforming layer using a chemical vapor deposition (CVD) method (Appl. Phys. Lett. 2002, 81, 1119-1121).
These methods, however, require high-cost vapor deposition equipment or vacuum equipment. Moreover, a polymer layer formed by a polymerization process, such as vapor deposition or plasma-polymerization, becomes physically and chemically unstable over time. Moreover, using such methods makes it difficult to chemically define the reforming layer and the storage life of the OLED is decreased because the physical and chemical stability of the reformed anode electrode becomes poor.